In analyzing stress in a specimen, two different pieces of information are important: the directions of principal stress and the magnitude of the difference in the stresses along the two principal directions. A polariscope is used to obtain this information. The polariscope includes a light source, a polarizer to linearly polarize the light from the source in a given plane, and an analyzer which transmits the component of light from the specimen which lies in a plane normal to the plane of polarization of the polarizer. The pattern developed at the analyzer can be focused onto a screen or exposed onto film.
The specimen may be a photoelastic model, or the specimen may be a photoelastic coating bonded fully or partially to the prototype.
When the specimen is under stress, the polariscope will depict two superimposed fringe patterns. One group, called isochromatic fringes, represents lines or contours along which the difference of principal stresses in the specimen is constant in magnitude. When white light is employed, the isochromatic fringes are colored bands. Also observed is an isoclinic fringe which is the locus of points in the specimen that have a principal-stress direction in the plane of polarization of the polarizer.
In order to obtain information on another isoclinic, the polarizer and analyzer are rotated together to different inclinations. The isoclinic fringe represents the locus of points that have a principal-stress direction in this new plane of polarization. This procedure is continually repeated until the desired number of isoclinics have been produced. Usually a photograph is taken of the pattern at each position of the polarizer/analyzer. This procedure is too time-consuming and expensive because of the need to change the orientations of the polarizer/analyzer and to take multiple photographs.